European Patent Application No. 0 420 396 A2 (Smith Kline & French Laboratories Limited) and Howson et al., Bioorg. & Med. Chem. Letters, Vol. 2 No. 1 (1992), pp. 77-78 describe imidazole derivatives having an amidine group as H.sub.3 agonists. Van der Groot et al. (Eur. J. Med. Chem. (1992) Vol. 27, pp. 511-517) describe isothiourea analogs of histamine as potent agonists or antagonists of the histamine H.sub.3 receptor, and these isothiourea analogs of histamine overlap in part with those of the two references cited above. Clapham et al. ["Ability of Histamine H.sub.3 Receptor Antagonists to improve Cognition and to increase Acetylcholine Release in vivo in the Rat", British Assn. for Psychopharmacology, Jul. 25-28 1993, reported in J. Psychopharmacol. (Abstr. Book), A17] describe the ability of histamine H.sub.3 receptor antagonists to improve cognition and to increase release of acetylcholine in vivo in the rat. Clapham et al. ["Ability of the selective Histamine H.sub.3 Receptor Antagonist Thioperamide to improve Short-term Memory and Reversal Leaming in the Rat", Brit. J. Pharm. Suppl., 1993, 110, Abstract 65P] present results showing that thioperamide can improve short-term memory and reversal learning in the rat and implicate the involvement of H.sub.3 receptors in the modulation of cognitive function. Yokoyama et al. ["Effect of thioperamide, a histamine H.sub.3 receptor antagonist, on electrically induced convulsions in mice", Eur. J. Pharmacol., vol. 234 (1993), pp. 129-133] report how thioperamide decreased the duration of each phase of convulsion and raised the electroconvulsive threshold, and go on to suggest that these and other findings support the hypothesis that the central histaminergic system is involved in the inhibition of seizures. International Patent Publication No. WO9301812-A1 (SmithKline Beecham PLC) describes the use of S-[3-(4(5)-imidazolyl)propyl] isothiourea as a histamine H.sub.3 antagonist, especially for treating cognitive disorders, e.g. Alzheimer's disease and age-related memory impairment. Schlicker et al. ["Novel histamine H.sub.3 receptor antagonists: affinities in an H.sub.3 receptor binding assay and potencies in two functional H.sub.3 receptor models"] describe a number of imidazolylalkyl compounds wherein the imidazolylalkyl group is bonded to a guanidine group, an ester group or an amide group (including thioamide and urea), and compare these to thioperamide. Leurs et al. ["The histamine H.sub.3 -receptor: A target for developing new drugs", Progr. Drug Res. (1992) vol. 39, pp. 127-165] and Lipp et al. ["Pharmacochemistry of H.sub.3 -receptors" in The Histamine Receptor, eds.: Schwartz and Haas, Wiley-Liss, New York (1992), pp. 57-72] review a variety of synthetic H.sub.3 receptor antagonists, and Lipp et al. (ibid.) have defined the necessary structural requirements for an H.sub.3 receptor antagonist.